Does a higher concentration of H⁺ make a solution more acidic or alkaline?
More acidic.
Does a higher concentration of OH⁻ make a solution more acidic or alkaline?
More alkaline.
What is the pH of a solution with a hydrogen ion concentration of 1 × 10⁻² mol dm⁻³?
pH = 2.
What is the pH of a solution with a hydroxide ion concentration of 1 × 10⁻³ mol dm⁻³?
pH = 11.
If a solution has a pH of 4, what is its pOH?
pOH = 10.
If a solution has a pOH of 6, what is its pH?
pH = 8.
What type of acid fully ionises in solution?
Strong acid.
What type of alkali partially ionises in solution?
Weak alkali.
What happens to the pH value when an acid is added to water?
The pH value decreases.
Explanation Questions
Explain how the concentration of hydrogen ions affects the pH of a solution.
An increase in hydrogen ion concentration leads to a decrease in pH, making the solution more acidic. This is because pH is defined as the negative logarithm of the hydrogen ion concentration, thus, the higher the H⁺, the lower the pH.
Explain how the concentration of hydroxide ions affects the pH of a solution.
An increase in hydroxide ion concentration leads to an increase in pH, making the solution more alkaline. As the concentration of OH⁻ increases, the concentration of H⁺ decreases, as described by the equation pH + pOH = 14.
Describe how a pH meter works and its advantages over other methods of pH measurement.
A pH meter is an electronic device that measures the electrical potential of a solution using an electrode, and this is then converted into a pH reading. It offers the advantage of being highly accurate and provides precise pH values, unlike indicator solutions or paper which provide approximate values.
How can a universal indicator be used to determine the pH of a solution?
Universal indicator is a mixture of indicators that changes colour over a wide range of pH values. By comparing the colour of the indicator in the solution to a standard colour chart, the approximate pH can be determined.
Explain the difference between a strong and weak acid in terms of pH.
A strong acid fully ionises in solution, releasing a large concentration of H⁺ ions, thus resulting in a lower pH. A weak acid only partially ionises, producing a smaller concentration of H⁺ ions, and a less extreme pH.
Explain the relationship between pOH and pH, and why both are used.
pH measures the acidity while pOH measures the alkalinity of a solution. The equation pH + pOH = 14 shows how these two values relate; if one value is known, the other can be calculated. Both are useful to describe a solution as they measure the concentration of H⁺ and OH⁻ ions respectively.
What does it mean if a solution is described as having a low pH, and give an example of this?
A low pH indicates a high concentration of hydrogen ions, which characterises an acidic solution. For example, gastric juice in the stomach has a low pH, around 1 or 2, due to the presence of hydrochloric acid.
What does it mean if a solution is described as having a high pH, and give an example of this?
A high pH indicates a high concentration of hydroxide ions, which characterises an alkaline or basic solution. For example, sodium hydroxide solution has a high pH around 13 or 14.
Explain how a substance like lemon juice can have an acidic pH when it contains water (which is neutral).
Lemon juice contains citric acid. When dissolved in water, the citric acid releases hydrogen ions, increasing the concentration of hydrogen ions and lowering the pH of the solution, making it acidic.
Explain why strong alkalis like sodium hydroxide are considered to be dangerous.
Strong alkalis fully ionise in solution. This means they have a very high concentration of hydroxide ions, resulting in a high pH. They are corrosive and can cause severe burns due to their chemical reactivity.